1. Field of the Invention:
The present invention relates to a superconductive magnet having a very low temperature refrigerator, and more particularly to the structure of a superconductive magnet which is capable of improving the cooling performance and the size of which can be reduced.
2. Description of the Related Art:
FIG. 19 is a cross sectional view which illustrates an example of a conventional superconductive magnet. Referring to FIG. 19, reference numeral 1 represents a superconductive coil. The superconductive coil 1 is immersed in liquid helium 3 enclosed in a helium chamber 2 serving as a very low temperature refrigerant chamber, the liquid helium 3 serving as a very low temperature refrigerant. As a result, the superconductive coil 1 is maintained at a very low temperature. Reference numeral 4 represents a vacuum chamber disposed to surround the helium chamber 2. A space between the vacuum chamber 4 and the helium chamber 2 is evacuated so that the two chambers 4 and 2 are thermally insulated.
Reference numeral 5 represents a second heat shield and 6 represents a first heat shield, the first and second heat shields 5 and 6 being so disposed between the helium chamber 2 and the vacuum chamber 4 to form coaxial cylinder shapes surrounding the helium chamber 2. As a result, heat invasion into the helium chamber 2 is reduced.
Reference numeral 7 represents a liquid nitrogen container formed in a portion of the first heat shield 6, the liquid nitrogen container 7 accommodating liquid nitrogen 8.
Reference numeral 9 represents, for example, a Gifford-MacMahon's two-stage refrigerator comprising a first heat stage 10 set to an absolute temperature of 80 K (Kelvin), a second heat stage 11 set to 20 K and a motor portion 12. The refrigerator 9 is, in the axial direction of the magnet, disposed downwards from an upper portion, the refrigerator 9 being so constituted that the first and second heat stages 10 and 11 respectively cool the first and second heat shields 6 and 5.
Reference numeral 13 represents a port portion disposed for the purpose of injecting the liquid helium 3 and inserting an electric-current lead for supplying electric current to the superconductive coil 1. Reference numeral 14 represents a cold bore.
The operation of the foregoing superconductive magnet will now be described.
The first heat shield 6 is cooled to 80 K by the liquid nitrogen 8 accommodated in the liquid nitrogen container 7 and the first heat stage 10 of the refrigerator 9. The second heat shield 5 is cooled to 20 K by the second heat stage 11 of the refrigerator 9. The invasion heat from outside is insulated in vacuum by the vacuum chamber 4 and shielded by the first and second heat shields 6 and 5 so that the heat invasion into the helium chamber 2 is reduced.
The superconductive coil 1 is cooled to a very low temperature (for example, 4.2 K) by the liquid helium 3 in the helium chamber 2 so that its superconductive state is maintained. When an exciting electric current is, in the foregoing state, supplied from an external power source (omitted from illustration) to the superconductive magnet via the electric current lead (omitted from illustration), a desired magnetic field is generated.
However, the fact that the foregoing conventional superconductive magnet is a hollow magnet facing sideways and the refrigerator 9 is vertically disposed from an upper portion in the axial direction of the magnet necessitates the length for which a piston, called a "displacer", reciprocates to be maintained in order to achieve the cooling performance of the refrigerator 9. Therefore, a large gap must be maintained between the first heat shield 6 and the second heat shield 5 and another large gap must be maintained between the vacuum chamber 4 and the first heat shield 6. As a result, the height of the apparatus cannot be shortened and the size of the same cannot be reduced.
Therefore, an applicant of the present invention has disclosed a superconductive magnet directed to overcome the foregoing problem and arranged such that a very low temperature refrigerator is disposed substantially horizontally and helium gas evaporated in the helium chamber is reliquefied (refer to Japanese Patent Publication No. 4-70922).
FIG. 20 is a partially broken away perspective view which illustrates the conventional superconductive magnet disclosed in Japanese Patent Publication No. 4-70922. Referring to FIG. 20, reference numeral 30 represents a three-stage regenerative type refrigerator disposed on the end surface of a vacuum chamber 4 in substantially parallel to the axial direction of a cylindrical superconductive coil 1. Reference numeral 31 represents an iron magnetic shied disposed to, together with an iron magnetic shield flange 32, surround the vacuum chamber 4. Reference numeral 33 represents a bore, 34 represents a discharge valve fastened to a port portion 13, 35 represents a fastening leg for the superconductive magnet and 36 represents a pressure controller unit for controlling the pressure in a helium chamber 2.
The superconductive magnet is formed into a hollow magnet facing sideways by coaxially disposing the helium chamber 2 for accommodating the superconductive coil 1, a second heat shield 5, a first heat shield 6 and a vacuum chamber 4.
The three-stage regenerative refrigerator 30 for use in the conventional superconductive magnet will now be described with reference to FIG. 21.
The three-stage regenerative refrigerator 30 is so arranged that a first displacer 16, a second displacer 17 and a third displacer 41 are slidably disposed in a cylinder 40 made of, for example, honing pipes and formed into three stages. Further, a first seal 18, a second seal 19 and a third seal 42 for preventing leakage of helium gas 24 are respectively disposed between the cylinder 40 and first, second and third displacers 16, 17 and 41. In addition, a first heat stage 10, a second heat stage 11 and a third heat stage 43 are disposed on the outer surfaces of corresponding stages of the cylinder 40.
A third regenerator 45 in the third displacer 41 is composed of a high-temperature portion 45a using GdRh exhibiting a large specific heat at 20 K to 7.4 K as a regenerating material and a low temperature portion 45b using Gd0.5Er0.5Rh exhibiting a large specific heat at temperature lower than 7.5 K as a regenerating material.
The three-stage regenerative refrigerator 30 is operated as follows:
First, high pressure helium gas 24 compressed by a helium compressor 25 serving as a helium gas compression means is introduced into first, second and the third expansion chambers 22, 23 and 46 in a state where the first, second and the third displacer 16, 17 and 41 are at the lowermost positions, a suction valve 26 is opened and an exhaust valve 27 is closed so that a high pressure state is realized.
Then, the first, second and the third displacers 16, 17 and 41 are moved upwards, and the high-pressure helium gas 24 is passed through the first, second and the third regenerators 20, 21 and 45 and introduced into the first, second and third expansion chambers 22, 23 and 46. During the foregoing process, the suction and exhaust valves 26 and 27 are not operated. The high-pressure helium gas 24 is cooled to a predetermined temperature by regenerating materials in the first regenerator 22, the second regenerator 23 and the third regenerator 45 when the helium gas 24 is passed through the foregoing regenerators 22, 23 and 45.
When the first, second and the third displacers 16, 17 and 41 are moved to the uppermost positions, the suction valve 26 is closed and the exhaust valve 27 is opened so that the high-pressure helium gas 24 is expanded in the low-pressure portion and refrigeration is generated. At this time, the helium gas 24 is formed into a low-temperature and low-pressure gas.
The ensuing downward movements of the first, second and the third displacers 16, 17 and 41 cause the low-temperature and low-pressure helium gas 24 to be passed through the first, second and the third regenerators 20, 21 and 45 before it is exhausted from the exhaust valve 27. The low-temperature and low-pressure helium gas 24, at this time, cools the regenerating materials in the first, second and the third regenerators 20, 21 and 45 before it is returned to the helium compressor 25.
In an ensuing state where the capacities of the first, second and third expansion chambers 22, 23 and 36 have been minimized, the exhaust valve 27 is closed and the suction valve 26 is opened. As a result, the high pressure helium gas 24 compressed by the helium compressor 26 is introduced so that the pressures in the first, second and the third expansion chambers 22, 23 and 46 are lowered from the high pressures.
The high pressure, for example, 20 bars-helium gas 24 is cooled to 60 K by the first regenerator 20, cooled to 15 K by the second regenerator 21 and cooled by the third regenerator 45 before it is introduced into the third expansion chamber 46.
If the regenerating material in the third regenerator 45 is lead, its specific heat is smaller than that of the helium gas 24. Therefore, the helium gas 24 is not cooled sufficiently but it is introduced into the third expansion chamber 46. As a result, the temperature of the expansion chamber is raised, resulting in a loss to be generated. In this case, an unsatisfactory temperature level of about 6.5 K can be realized. If GdRh is used as the regenerating material, its specific heat is larger than that of lead. Therefore, the loss can be restricted and, accordingly, a satisfactory temperature level of 5.5 K can be realized.
If the regenerating material comprises GdRh and Gd0.5Er0.5Rh (the weight ratio of GdRh is made to be 45 to 65%), a temperature level of 4.2 K can be realized. When the surface roughness of the inner surface of the cylinder 40 was made to be 0.5 .mu.m RMS to reduce the leakage through the sealed portion, a temperature level of 3.68 K was realized.
When Er3Ni was used as the regenerating material in place of GdRh, a similar temperature level was realized.
It should be noted that the "high pressure" for the helium gas 24 was set to 20 bars and the "low pressure" was set to 6 bars.
Since the three-stage regenerative refrigerator 30 is constituted by the first regenerator 20 using the copper net as the regenerating material, the second regenerator 21 using lead balls as the regenerating material, the third regenerator 45 composed of a high temperature portion 45a using GdRh as the regenerating material and a low temperature portion 45b using Gd0.5Er0.5Rh as the regenerating material, excellent refrigerating performance can be obtained such that the first heat stage 10 realizes 50 to 80 k, the second heat stage 11 realizes 10 to 20 K and the third heat stage 43 realizes 2 to 4.5 K. Therefore, the superconductive magnet can be operated stably.
FIG. 22 illustrates the fastened state of the structure of the three-stage regenerative refrigerator 30. An L-shape pipe 50 made of stainless steel and serving as an outlet portion is so fastened to the upper portion of the helium chamber 2 that an end of the L-shape pipe 50 faces the helium gas atmosphere evaporating in the helium chamber 2. Further, a three-stage cylinder 51 for fastening the refrigerator 30 and made of stainless steel is fastened to the end surface of the vacuum chamber 4, the cylinder 51 for fastening the refrigerator 30 being fastened substantially in parallel to the axial direction of the superconductive coil 1. The L-shape pipe 50 and the cylinder 51 for fastening the refrigerator 30 are connected to each other by bellows 52. The cylinder 51 for fastening the refrigerator 30 has a copper first stage 53 and a second stage 54 which are respectively thermally connected to a first heat shield 6 and a second heat shield 5.
The three-stage regenerative refrigerator 30 is fastened as follows: the third heat stage 43 is so inserted into the cylinder 51 for fastening the refrigerator 30 that the third heat stage 43 is exposed to the helium gas atmosphere received into the L-shape pipe 50; and the first heat stage 10 and the second heat stage 11 are thermally connected to the cylinder 51 for fastening the refrigerator 30.
Since the cylinder 51 for fastening the refrigerator 30 is fastened at the end surface of the vacuum chamber 4 to substantially run parallel to the axial direction of the superconductive coil 1 as described above, the reciprocative movement distance for each displacer contributing to the refrigerating performance of the three-stage regenerative refrigerator 30 can be maintained while eliminating necessities of enlarging the gaps among the helium chamber 2, the second heat shield 5, the first heat shield 6 and the vacuum chamber 4. As a result, the size of the superconductive magnet can be reduced. Further, the arrangement that the three-stage regenerative refrigerator 30 is detachably fastened to the cylinder 51 for fastening the refrigerator 30 enables the three-stage regenerative refrigerator 30 to be removed without decomposing the apparatus. As a result, the maintenance facility can be improved.
FIG. 23 illustrates the thermal connections established among the cylinder 51 for fastening the refrigerator 30, the first heat shield 6 and the second heat shield 5. The second heat shield 5 has a second cut portion 60 formed therein. Further, the first heat shield 6 has a first cut portion 61 so arranged that the second cut portion 60 appears. In addition, the vacuum chamber 4 has a cut portion 62 so arranged that the first cut portion 61 appears.
By establishing connections between the first stage 53 and the first heat shield 6 and between the second stage 54 and the second heat shield 5 by making use of flexible conductors 63 each of which is manufactured by, for example, knitting copper wires, the first stage 53 and the first heat shield 6 are thermally connected to each other. Similarly, the second stage 54 and the second heat shield 5 are thermally connected to each other. Further, a second radiation cover 55 and a first radiation cover 56 are disposed to cover the second cut portion 60 and the first cut portion 61, respectively. In addition, a capping plate 57 made of stainless steel is fastened to the vacuum chamber 4 to cap the cut portion 62.
The first stage 53 and the second stage 54 of the cylinder 51 for fastening the refrigerator 30, end portions of which are fastened to the end surface of the vacuum chamber 4 and other end portions of which are fastened to the L-shape pipe 50 while interposing the bellows 52, are structured as to appear in the cut portion 62 and the first cut portion 61. As a result, the first stage 53 and the second stage 54 can easily establish the thermal connections among the cylinder 51 for fastening the refrigerator 30, the first and second heat shields 6 and 5 in such a manner that the connections are not hindered by the first and second heat shields 6 and 5 and the vacuum chamber 4. Further, the arrangement that the first and the second cut portions 61 and 60 are capped by the first and the second radiation covers 56 and 55 reduces the external heat invasion.
FIG. 24 illustrates the connection structure established between the three-stage regenerative refrigerator 30 and the cylinder 51 for fastening the refrigerator 30. The cylinder 51 for fastening the refrigerator 30 has, on the inner surface thereof on which the first stage 53 is fastened, a heat conductor 64 adjacent to the cylinder 51 for fastening the refrigerator 30, the conductor 54 having a tapered surface. The first heat stage 10 of the three-stage regenerative refrigerator 30 has a heat conductor 65 adjacent to the refrigerator 30, the heat conductor 65 having a tapered surface in which knurling is formed to face the tapered surface of the heat conductor 64 adjacent to the cylinder 51 for fastening the refrigerator 30.
Further, the heat conductor 64 adjacent to the cylinder 51 for fastening the refrigerator 30 and the heat conductor 65 adjacent to the refrigerator 30 respectively are disposed on the inner surface of the cylinder 51 for fastening the refrigerator 30, at which the second stage 54 is fastened, and the second heat stage of the third stage regenerative refrigerator 30. The heat conductor 64 adjacent to the cylinder 51 for fastening the refrigerator 30 and the heat conductor 65 adjacent to the refrigerator 30 are made of copper which is excellent heat conductive material.
An indium wire 66, which is soft metal for establishing the thermal connection, is disposed between the heat conductor 64 adjacent to the cylinder 51 for fastening the refrigerator 30 and the heat conductor 65 adjacent to the refrigerator 30. Further, a bolt 69 for fastening a flange 68 of the three-stage regenerative refrigerator 30 is, via a belleville spring 70, which is an elastic member, fastened to a fastening flange 67 of the cylinder 51 for fastening the refrigerator 30. In addition, an "O" ring 71 which is an airtightening sealing member is disposed between the fastening flange 67 and the flange 68.
When the flange 68 is fixed to the fastening flange 67 by making use of the bolt 69, the flange 68 slides while maintaining the airtightness thanks to the "O" ring 71. The fastening force of the bolt 69 plastically deforms the indium wire 66 so that the thermal connection is established between the heat conductor 64 adjacent to the cylinder 51 for fastening the refrigerator 30 and the heat conductor 65 adjacent to the refrigerator 30.
Excessive fastening force of the bolt 69 and the displacements of the elements due to the thermal contraction and vibrations are absorbed by the belleville spring 70 so that the breakage of the elements and the defective thermal connection can be prevented. Further, even if the three-stage regenerative refrigerator 30 is contracted after it has been fastened to the cylinder 51 for fastening the refrigerator 30 and cooled sufficiently, further tightening of the bolt 69 enables a desired fastening force to be maintained.
Further, the tapered surface of the heat conductor 65 adjacent to the refrigerator 30 is knurled so that the fastening force between the indium wire 66 and the knurled surface is enlarged. Therefore, when the three-stage regenerative refrigerator 30 is removed, the removal can be performed such that the indium wire 66, which has been deformed plastically, adheres to the tapered surface of the heat conductor 65 adjacent to the refrigerator 30.
Since the conventional superconductive magnet has the arrangement that the refrigerator 9 is disposed vertically in the axial direction of the magnet as described above, the position of the magnet device cannot be lowered and, accordingly, the overall size cannot be reduced.
Since the superconductive magnet disclosed by the applicant of the present invention has the arrangement that the three-stage regenerative refrigerator 30 is disposed substantially horizontally as to reliquefy the helium gas evaporated in the helium chamber 2, the height of the apparatus can be lowered and therefore the overall size of the apparatus can be reduced. Further, the distance for which the displacer reciprocates can be maintained, causing the refrigerating performance to be improved. However, a gap is formed between the cylinder 51 for fastening the refrigerator 30 and the three-stage regenerative refrigerator 30, the gap being filled with the helium gas received through the L-shape pipe 50 and evaporated in the helium chamber 2. Further, a thermal gradient occurs in each stage of the cylinder 40 of the three-stage regenerative refrigerator 30. The foregoing helium gas is heated in the hot portion in the cylinder 40, cooled in the low temperature portion, and therefore a heat convection is generated. It leads to a fact that the temperature of the first heat stage 10, which is the low temperature portion, is raised. Therefore, there arises a problem in that the cooling performance of the refrigerator deteriorates.